Issue 31

R.D.S.G. Campilho et alii, Frattura ed Integrità Strutturale, 31 (2015) 1-12; DOI: 10.3221/IGF-ESIS.31.01 7 Figure 5 : Image after applying the Difference of Gaussian filters and the extracted lines ( l 1 and l 2 ) that are used for measuring  o . Then, for the rows of the image between p 5 and p 6 , the midpoint of the edge at each row is computed. The midpoint is first extracted for the row of p 5 by (1) collecting all the pixels to the left and to the right of p 5 that can be reached from p 5 without dropping the pixel intensity bellow a given threshold (10% for all experiments), and (2) by averaging the position of all the collected pixels weighted by the pixels intensity value (so that pixels with higher intensities, i.e. pixels belonging to the edge, have a higher impact in the row’s midpoint calculation). This makes the process robust to blur in the images and to the point identification process because points p 5 and p 6 do not need to be identified exactly in the midline of the edge. This process is repeated for all the following rows until reaching p 6 , resulting in one point per row of the image between p 5 and p 6 that define the midline of the edge of the ruler. Since these points are not necessarily collinear, a linear regression is used for obtaining l 1 . The same process is repeated with points ( p 7 , p 8 ) for obtaining l 2 and, finally,  o may be calculated as the angle between the two lines 1 2 0 1 2 arccos v v v v                (7) where 1 v  and 2 v  are the direction vectors of lines l 1 and l 2 , respectively. Fig. 6 shows the  o -testing time plot for a specimen, more specifically the three curves of Fig. 4. Due to scaling difficulties, the raw curve in the figure is already translated such that  o (testing time=0)=0.  o = 1.0646E-11 t 4 - 4.6515E-09 t 3 + 8.0223E-07 t 2 + 7.7256E-05 t + 8.6406E-03 R² = 9.9726E-01 0 0.02 0.04 0.06 0 50 100 150 200 250 300  o [rad] testing time, t [s] Raw curve Adjusted curve Polinomial (Raw curve) y i l ( r ) Figure 6 : Evolution of  o for one test specimen: raw curve obtained from the optical method, polynomial fitting curve and corrected polynomial curve.